1. Field of the Invention
The present invention relates to a light limiter, and in particularly, to a light limiter removing the high output signal light component of pulses (abbreviated as an optical surge) secondarily generated in an optical amplification process.
This application is based on patent application No. 10-020010 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Recently, in order to optically amplify a signal light directly and to improve the transmission distance of the signal light, optical fiber amplifier whose cores are doped with rare earth ions such as Er.sup.3+ and Nd.sup.3+ (an optical fiber amplifier an alias) are required and essential in an optical communication system which can communicate in a long distance and has a mass storage.
However, the optical fiber amplifier outputs high power light; therefore, it may degrade or destroy the optical components located on a transmission line in a next step. This may be a primary factor for the degradation of the stability and reliability of an optical communication system.
In particular, secondary generating optical surges are one of the primary degradation factors of the optical components in the optical amplification process in which the signal light starts to be input to the optical fiber amplifier. When the optical fiber amplifier is excited by an exciting light having shorter wavelength than that of the signal light, and the signal light does not input thereto, the rare earth ions of high excitation level are stored in the optical fiber amplifier. When the signal light input to the optical fiber amplifier in this condition, the stored high level energy is suddenly undergoes induced-emission.
In order to prevent these optical surges, the following methods have been suggested.
FIG. 15 shows the optical fiber amplifier disclosed in Japanese Patent Application, First Publication No. Hei 06-216452. This optical fiber amplifier comprises an optical multiplexer 1, a semiconductor laser emitting apparatus 2 which emits dummy-light, a control means 3 for the semiconductor laser emitting apparatus 2, a light receiving device 4, an optical branching device 5, an optical multiplexer 6 for a multiplication of the signal light and the dummy-light, a semiconductor laser emitting apparatus 7 emitting an excited light, a driving circuit 8 for a semiconductor laser emitting apparatus 7, optical isolators 9, 11, a light amplification portion 10 of rare-earth element doped optical fiber, and an optical filter 12.
When the input signal light S.sub.1 having a wavelength of .lambda..sub.1 is amplified in this optical fiber amplifier, the input signal light S.sub.1 is combined with the dummy-light S.sub.2 having a wavelength of .lambda..sub.2 being different from the wavelength .lambda..sub.1 of the input signal light S.sub.1 before an optical amplification process. Then, the power of the dummy-light S.sub.2 is controlled so that the total amount of power between the input signal light S.sub.1 and the dummy-light S.sub.2 is fixed. A part of the transmission power of the composed light is branched by the optical branching device 5, and is subjected to an optical/electrical conversion (O/E conversion) by the light receiving device 4. The residue of the composed light is input to the optical multiplexer 6. The control means 3 for the semiconductor laser emitting apparatus 2 which emits dummy-light feeds back to the semiconductor laser emitting apparatus 2 which emits dummy-light so as to fix the voltage value obtained by the O/E conversion. The excited light S.sub.3 having a wavelength of .lambda..sub.3 output from the semiconductor laser emitting apparatus 7 emitting an excited light, the input signal light S.sub.1, and the dummy-light S.sub.2 are combined in the optical multiplexer 6. The combined light inputs to the light amplification portion 10 of a rare-earth element doped optical fiber, via the optical isolator 9. The light amplification portion 10 of rare-earth element doped optical fiber is excited by the excited light S.sub.3. Thereby the input signal light S.sub.1 and the dummy-light S.sub.2 are amplified in the light amplification portion 10 of rare-earth element doped optical fiber, the dummy-light S.sub.2 is removed by the optical filter 12, and then only the amplified input signal light S.sub.1 is output.
When the input signal light S.sub.1 is not input into the optical fiber amplifier, the energy stored in the light amplification portion 10 of the rare-earth element doped optical fiber is suddenly subjected to induced-emission; therefore, the optical surges are generated in this optical fiber amplifier. Accordingly, an optical surges are prevented by the incidence of the dummy-light S.sub.2 to the light amplification portion 10 of the rare-earth element doped optical fiber when the input signal light S.sub.1 is not input, and by control the light intensity of the input signal light S.sub.1 and the dummy-light S.sub.2 is fixed.
The light input to the a light amplification portion 10 of rare-earth element doped optical fiber is subjected not to cut off, in order to prevent the output of an optical surge in this conventional optical fiber amplifier. Therefore, the optical surge is not restricted. Accordingly, this conventional optical fiber amplifier has the problem that control the power of the generated optical surge is impossible. In addition, this conventional optical fiber amplifier has many components; therefore, its reliability may deteriorate.
In particular, the optically active components, such as a laser photogenic organ, have a worse reliability parameter (called the FIT value). It is preferable that the generation of the dummy-light which is not essential be prevented, and the number of the components is reduced in a undersea repeater which is preferably has a higher grade of reliability than that of a land-based repeater.